1. Field of the Invention
The present invention relates to a flat configuration cathode ray tube (hereinafter abbreviated to CRT), and in particular to an improved flat configuration CRT of a type in which electron beam modulation is executed by beam control electrodes which are disposed behind and closely adjacent to an array of line cathodes used as an electron beam source.
2. Background of the Prior Art
A flat configuration CRT of a type employing beam control electrodes positioned behind an array of line cathodes (i.e. positioned on the opposite side of that array of cathodes from the direction of emission of electron beams derived therefrom) is described in the prior art, for example in Japanese Patent Laid-open No. 63-37938 and 56-79845. With such a CRT, a plurality of line cathodes and a first grid electrode having an array of through-holes formed therein are disposed mutually opposing. These through-holes are arranged in a plurality of horizontal rows, i.e., each row being positioned in correspondence with one of the line cathodes, for forming a row of electron beams from electrons which are emitted from the corresponding cathode. These electron beams then pass through deflection and acceleration electrodes, to be directed onto a fluorescent layer formed on a transparent faceplate of the CRT. In addition, a plurality of elongated electron beam control electrodes extending at right angles to the line cathodes are positioned behind the line cathodes, for controlling the respective intensities of the electron beams in accordance with signal voltages applied to these beam control electrodes, i.e. for modulating the electron beams in accordance with the contents of a video signal, to thereby display a corresponding picture. The set of beam control electrodes also function to direct the electrons emitted from each line cathode in the required direction for electron beam generation.
A typical configuration of such a prior art flat configuration CRT will be described referring to FIG. 1. Here, numeral 1 denotes line cathodes each formed of a high melting-point metal wire such as tungsten wire, and having a electron emission material coated on the surface thereof. A plurality of these line cathodes are held in tension, mutually parallel, extending in the horizontal direction, and are heated to obtain emission of electrons (where the terms "horizontal" and "vertical" directions as used herein signify directions respectively parallel to the horizontal and vertical directions of a picture displayed by the CRT). Numeral 2 denotes a set of elongated vertically extending electron beam control electrodes, and numeral 12 denotes a set of elongated vertically extending shield electrodes. The shield electrodes 12 and electron beam control electrodes 2 are formed at successively alternating positions on an electrically insulating substrate 3, which is formed of a material such as glass or ceramic. Generally, a portion of a glass envelope of the CRT can be used as the electrically insulating substrate 3. A fixed spacing is established between the array of line cathodes 1 and the array of electron beam control electrodes 2. Each of the line cathodes 1 is retained under tension by a spring (not shown in the drawings) attached to at least one end thereof. Numerals 4 and 7 denote first and second grid electrodes for respectively forming and focussing the electron beams, having arrays of through-holes 5 and 8 respectively formed therein, arranged in rows which are oriented parallel to and positioned in correspondence with respective ones of the line cathodes 1. The dimensions of the through-holes 5 and 8 are determined by the requisite electron beam size and the position relationships of the various electrodes.
The electron beam forming grid electrode 4 has the through-holes 5 formed therein at positions which correspond to respective positions of intersection between the electron beam control electrodes 2 and the line cathodes 1. Numeral 6 denotes vertical deflection electrodes for deflecting the electron beams in the vertical direction. The apertures 8 in the grid electrode 7 are of elongated shape and correspond in horizontal position to the through-holes 5 that are formed in the electron beam forming grid electrode 4. The grid electrode 7 serves to shield the vertical deflection electrode 6 from the effects of a high electric field that results from a high voltage applied to a metal back electrode 9 (described hereinafter). A transparent faceplate 11 has an electroluminescent layer 10 formed on an inner surface thereof and also has a metal back electrode 9 formed thereon.
With such a CRT, since the electron beam control electrodes are closely mutually adjacent, the control characteristics of each of these electrodes may be affected by changes in potential of adjacent ones of the electron beam control electrodes, i.e., there may be crosstalk. To prevent such crosstalk, a set of shield electrodes 12 can be utilized as shown in the oblique view of FIG. 2. As shown, the shield electrodes 12 are formed on the electrically insulating substrate 3 at positions which alternate with those of the electron beam control electrodes 2, with all of the shield electrodes 12 being mutually electrically connected at one end thereof (to which a fixed DC voltage is applied). Fixed spacings are established between the electron beam control electrodes 2, the line cathodes 1 and the shield electrodes 12, with these elements being placed as mutually closely as possible.
The operation of the prior art example of FIG. 3 is as follows. Each of the electron beam control electrodes 2 is connected through one of a set of resistors r1, r2, r3, . . . . . to a negative bias voltage source V.sub.1. One end of each of the line cathodes 1 is connected through a respective one of a set of resistors R1, R2, R3, . . . . . to a positive bias voltage source V.sub.2, while the other ends of the line cathodes 1 are connected through respective ones of a set of diodes D.sub.1, D.sub.2, D.sub.3, . . . . . . to the negative bias voltage source V.sub.2. A positive voltage is applied to the electron beam forming grid electrode 4 from a voltage source V.sub.3. The line cathodes 1 are normally connected to receive a current flow from the voltage source V.sub.2, for heating. However once in each vertical scanning interval, when a cathode is to be utilized to derive a row of electron beams during a fixed interval, a negative voltage pulse is applied (from the corresponding one of a set of terminals A1, A2, A3, . . . ) to that cathode to thereby halt the flow of heating current through the cathode and also to bias the cathode in a direction tending to enable electron emission therefrom. In this condition, if a positive voltage pulse is applied to one of the beam control electrodes 2 (i.e. from a corresponding one of the input terminals B1, B2, B3, . . . . ), then the inhibiting effect of a negative voltage normally applied to that beam control electrode through the corresponding one of the resistors r1, r2, r3, . . . . . from the voltage source V1 will be removed for the duration of that pulse, and a high level of electron emission from that cathode will occur.
In this way, a row of modulated electron beams can be derived from the corresponding row of apertures in the first grid electrode 4. Thus for example by applying respective pulse-width modulated signals in parallel to the input terminals B1, B2, B3, . . . . in accordance with the contents of a video signal, the electron beams can be modulated in accordance with that video signal, to thereby display a corresponding picture by the CRT. The line cathodes 1 are successively switched to the "emission possible" condition by the negative pulses applied to the terminals A1, A2, A3 . . . . . for a fixed interval during each vertical scanning interval, sequentially from the uppermost to the lowermost cathode.
The electron beams that are thus selectively transferred through the through-holes 5 formed in the electron beam forming grid electrode 4, then are deflected by the vertical deflection electrodes 6, to be then accelerated by the high voltage that is applied between the grid electrode 7 and the metal back electrode 9, to thereby impinge upon the electroluminescent layer 10 of the image display faceplate 11 and so produce a display picture.
However with such a prior art flat configuration image display apparatus, due to the fact that the separation between the line cathodes 1 and the electron beam control electrodes 2 is very small, even a slight change in that separation will result in a large change in the electron beam current that is derived from the line cathodes 1. As a result, a very high degree of accuracy is necessary for the flatness of the insulating substrate 3 on which the electron beam control electrodes are formed, and also for the spacing between the line cathodes 1 and the electron beam control electrodes 2. Thus, a prior art CRT of this type presents serious problems with regard to ease of manufacture and manufacturing yield.
Moreover with prior art examples of this type of flat configuration CRT in which shield electrodes 12 are incorporated with a fixed voltage being applied in common to all of the shield electrodes, there is no description of how the value of that fixed voltage can be optimized such as to provide a maximum level of electron beam current together with effective shielding of mutually adjacent electron beam control electrodes.